Short-term solution could be to remove the Title typehint from $title (which was added in https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/AbuseFilter/+/460335/). However, it wouldn't actually do much, since our code isn't really meant to handle filtering without a title.
Proper and long-term solution is to ensure filtering only happens with a real title. This may be done either by halting AF execution if the title is null, or changing the used hook to make sure it always passes a title. Anyway, this only happens with uploads, and I guess only for uploads where an invalid title is specified.
See T144265 (and related patches) for the progress in the long-term.

@Daimona This is still one of the most common fatal errors, presumably responsible for edits and/or uploads by users wrongly being aborted or prevented. Can you confirm that the patch causing it can be cleanly reverted and/or submit an alternate patch that I can review instead?

@Krinkle AFAICS The fatal only happens for uploads, specifically for uploads where the specified title isn't valid. As I was saying above, avoiding the fatal doesn't avoid silent errors, but I agree that it'd be less intrusive for users. I can't send a patch right now, but the one above is definitely an overkill. All we need to do is remove the "Title" typehint from line 1100. Also, could you please take a look at https://gerrit.wikimedia.org/r/466842?